Given a known high-performance printer having only a single printing unit, the individual printer components are exactly matched to one another, so that a very reliable operating condition with high printing output is achieved. In order to increase the printing output even farther, structural modifications must be undertaken at these components, this leading to a high development outlay and raising the outage probability due to the redesign of the individual components.
U.S. Pat. No. 4,587,532 discloses a printer that has three printing units that print single sheets in common or alternation arranged above one another. The paper delivery contains shunts in order to supply the single sheets to the various printing units.
DE 34 22 942 A1 discloses a printing system that has two electrophotographic printing stations. Given occurrence of a malfunction in one printing station, the appertaining printing station is shut off, whereby the other printing stations continues operating.
WO 91/13386 discloses a modularly constructed printer means that contains two printing modules. The printing modules can operate in simplex mode, whereby single sheets are supplied to both printing modules via shunts. Further, a duplex mode is possible, whereby the single sheets are first supplied to the one printing module for printing with a first print image; subsequently, a further print image can be applied with the second printing module. The printing modules are designed as separate structural units for installation in a single housing.
DE 34 07 847 A1 discloses an image recording means with multiple functions. Functions such as, for example, high-speed recording, two-side recording and multiplex recording are possible. The printer system contains two printing units that are connectable to one another via a paper guidance system. The paper guidance system also enables the single sheets to be turned over, so that a duplex mode with printing on both sides is possible.
U.S. Pat. No. 4,972,236, further, discloses an image generating system having a plurality of printing units that are arranged vertically over one another in order to save space. In one exemplary embodiment, the paper transports for both printing units are connected to one another by a connecting channel.
U.S. Pat. No. 5,150,167 A discloses a printing system that contains essentially identically constructed printing units. Each of the printing units has a separate input section allocated to it via which sheet material is supplied to the respective printing unit. Given outage of one of the printing units, for example given a paper jam within the transfer printing zone of a printing unit, the second printing unit must be served by the appertaining paper input compartment. What is disadvantageous given this arrangement is that two input compartments must be provided in order to maintain the operational readiness of the apparatus given the outage of a printing unit. When, for example, the lower printing unit goes down because of a paper jam and when the appertaining input compartment in the upper printing unit is empty, then the printer arrangement is entirely out of service.
U.S. Pat. No. 4,591,884 A and U.S. Pat. No. 5,208,640 A disclose a number of printing unit versions. However, a multi-functional arrangement such that both a simplex operating mode with high throughput as well as a duplex operating mode wherein two printing units are employed and an emergency mode given outage of one of the two printing units is not provided.
Therefore, there is a need for a method for the operation of a printer or of a copier that enables a high printing output or, respectively, copier output and works dependably.